1. Field of the Invention
This invention relates generally to oilfield equipment, and in particular to surface-mounted reciprocating-beam sucker rod pumping units, commonly referred to as pump jacks. More particularly still, the invention relates to pump jacks for producing wells having inclined wellheads.
2. Background Art
Hydrocarbons are often produced from well bores by reciprocating downhole pumps that are driven from the surface by pumping units. A pumping unit is connected to its downhole pump by a rod string. Although several types of pumping units for reciprocating rod strings are known in the art, walking beam style pumps enjoy predominant use due to their simplicity and low maintenance requirements.
FIG. 1 shows a class 1 walking beam pump jack (10) of prior art. The pump jack (10) is driven by a prime mover (12), typically an electric motor or internal combustion engine. The rotational power output from the prime mover (12) is typically transmitted by a belt or chain (14) to a gearbox (16). The gearbox (16) provides low-speed high-torque rotation of a crankshaft (22). Each end of the crankshaft (22) (only one is visible in FIG. 1) carries a crank arm (20) and a counterbalance weight (18). The reducer gearbox (16) sits atop a pedestal (17), which provides clearance for the crank arms (20) and counterweights (18) to rotate. The gearbox pedestal (17) is mounted atop a base (11). The base (11) also supports a samson post (13). The top of the samson post (13) acts as a fulcrum that pivotally supports a walking beam (24) via a saddle bearing assembly (15), commonly referred to as a center bearing assembly.
Each crank arm (20) is pivotally connected to a pitman arm (26) by a crank pin bearing assembly (19). The two pitman arms (26) are connected to an equalizer bar (27), and the equalizer bar (27) is pivotally connected to the rear end of the walking beam (24) by an equalizer bearing assembly (25). A horse head (28) with an arcuate forward face (29) is mounted to the forward end of the walking beam (24). The face (29) of the horse head (28) includes one or more tracks or grooves for carrying a flexible wire rope bridle (30). At its lower end, the bridle (30) terminates with a carrier bar (31), upon which a polished rod (32) is suspended. The polished rod (32) extends through a packing gland or stuffing box (34) at the wellhead (9). A rod string (36) of sucker rods hangs from the polished rod (32) within a tubing string (38) located within the well casing (40). The rod string is connected to the plunger of a subsurface pump (not illustrated). In a reciprocating cycle of the pump jack (10), well fluids are lifted within the tubing string (38) during the rod string (36) upstroke.
A walking beam pump jack operates, in essence, as a simple kinematic four-bar linkage mechanism, in which each of four rigid links is pivotally connected to two other of the four links to form a closed polygon. In a four-bar linkage mechanism, one link is typically fixed, with the result that a known position of only one other body is determinative of all other positions in the mechanism. The fixed link is also known as the ground link. The two links connected to the ground link are referred to as grounded links, and the remaining link not directly connected to the fixed ground link is referred to as the coupler link. Four-bar linkages are well known in mechanical engineering disciplines and are used to create a wide variety of motions with just a few simple parts.
Referring to FIG. 1, a four-bar linkage is embodied in the design of the pump jack (10) as follows: A fixed link (Link K) extends from the centerline of the crankshaft (12) to the centerline of the center hearing (15). Link K is defined by a grounded frame formed of interconnected rigid bodies including the samson post (13), the base (11), the gearbox pedestal (17), and the reducer gearbox (16). The first grounded link (Link R) is defined by the crank arms (20), and the second grounded link (Link C) is defined by the rear portion of the walking beam (24) extending from the centerline of the center bearing (15) to the centerline of the equalizer bearing (25). The pitmans (26) and the equalizer (27) together define the coupler link (Link P). This four-bar linkage is dimensioned so as to convert rotational motion of Link R into pivotal oscillation of Link C via the coupler Link P and the fixed Link K. That is, the crank arms (20) seesaw the walking beam (24) about the center bearing (15) atop the samson post (13) via the pitman arms (26) and equalizer (27).
Substantially all of the operating characteristics of a pump jack are determined by the dimensions of its four-bar linkage. For example, the torque factor relationship, polished rod position, stroke length, and counterbalance phase angle are dependent on the four-bar linkage dimensions. Torque factors and counterbalance phase angle are important parameters used to define the load carrying capacity of the pump jack. The varying interaction of these two terms with polished rod position is used to define permissible polished rod load envelope curves that are compared with measured dynamometer load data to verify that the reducer gearbox is operating within the designed torque loading.
The determination of pump jack operating characteristics is greatly simplified by the American Petroleum Institute (“API”) Specification 11E (“Specification for Pumping Units”). API Specification 11E includes derived operational parameters as a function of the geometry of a pumping unit's four-bar linkage, expressed in terms of standardized geometry designations. Accordingly, pump jacks are commonly specified in terms of the API geometry designations, and nearly all pump jack manufacturers provide these API geometry dimensions.
FIGS. 2A and 2B illustrate the geometry designations promulgated by API for class 1 lever and class 3 lever pump jacks, respectively. Dimension “A” is the distance from the center of the saddle bearing to the centerline of the polished rod. Dimension “C” is the distance from the center of the saddle bearing to the center of the equalizer bearing. Dimension “P” is the effective length of the pitman arm as measured from the center of the equalizer bearing to the center of the crank pin bearing. Dimension “R” is the distance from the centerline of the crankshaft to the center of the crank pin bearing. Dimension “H” is the height from the center of the saddle bearing to the bottom of the pump jack base. Dimension “I” is the horizontal distance from the center of the saddle bearing to the centerline of the crankshaft. Dimension “G” is the height from the centerline of the crankshaft to the bottom of the pump jack base. Finally, dimension “K” (FIG. 1) is the distance from the centerline of the crankshaft to the center of the saddle bearing. Dimension “K” may be computed as:K=√{square root over ((H−G)2+I2)}  (Equation 1).
Pump jacks, like pump jack (10) of FIG. 1, are typically designed to operate in conjunction with a vertically aligned wellhead (9). However, an increasingly common practice in drilling and production is for the well bore to be inclined at some non-vertical angle so that the well bore penetrates the fluid producing strata along a lengthened path, thus providing the well bore with greater exposure to the producing formation. Directional drilling allows wells to be completed down hole at angles up to and including 90 degrees from vertical.
Depending on the well depth, it may be necessary that the wellhead is also inclined relative to the vertical axis. Such is often the case in shallow wells with near horizontal downhole completion angle or when surface topology prohibits drilling the well from directly above the producing formation. The range of surface inclination typically varies between 0 and 45 degrees from vertical.
Non-vertical wellheads present problems for traditional surface-deployed sucker rod pumping units, because, from both a polished rod load and counterbalance (gravitational) alignment standpoint, pump jack design is based upon a fundamental assumption of vertical operation. This assumption has greatly influenced placement and orientation of structural members, working angles of articulation for the walking beam and horse head, and the phase angle of the crank-mounted counterbalance.
Referring to FIG. 3, U.S. Pat. No. 4,603,592, issued to Seibold et al. (“Seibold”), discloses one potential means of addressing an inclined wellhead with a modified pumping unit (10′) of the class 1 lever type. Seibold teaches adjustably lengthening the pitman arms (26′), tilting the samson post (13′), and enlarging the horse head (28′) so that the pumping unit (10′) can address wellheads (9′) of various inclinations. The effective length of the pitman arm (Link P′) and the rear span (Link C′) of the walking beam are increased to produce the desired angle bias. That is, Seibold approaches the problem of wellhead inclination by altering the four-bar linkage geometry so that the polished rod (32) aligns with the inclined wellhead (9′).
However, because the four-bar linkage is altered, these modifications have a significant effect on the operating characteristics of the pumping unit (10′). Modifications to the pumping unit four-bar linkage generally raise or lower the allowable polished rod load, change the shape of the permissible load envelope, alter the length of the pumping stroke, and induce a phase angle shift in the counterbalance. The polished rod speed and acceleration profiles are also sometimes substantially altered by these modifications.
Moreover, many downstream well analysis programs, diagnostic algorithms, rod pump controllers, and application tools involved in rod pump operation incorporate assumptions based upon standard four-bar linkage (K-R-P-C) usage into their calculations. While it is possible to predict the consequences of a modified linkage (K-R-P′-C′) and make adjustments as per Seibold's recommendations, the end user of the equipment is burdened with a more complex scenario with regard to proper application of the equipment.
Additionally, the prior art Seibold pump jack of FIG. 3—with elongated pitman arms walking beam and horse head likely requires more steel than an ordinary pump jack. It is desirable, therefore, to have a pump jack suitable for pumping at inclined wellheads that employs a standard four-bar linkage arrangement.
3. Identification of Objects of the Invention
A primary object of the invention is to provide a method and beam pump apparatus arranged for pumping wells having inclined wellheads in which the four-bar linkage geometry of the pumping unit remains unchanged relative to the standard pumping unit geometry.
Another object of the invention is to provide a method and beam pump apparatus for properly addressing an angled wellhead while leaving the operational characteristics of the pumping unit, the allowable loading envelope, and the motion profile the same as a vertically aligned pumping unit of the same linkage geometry.
Another object of the invention is to provide a method and beam pump apparatus having a modified forward walking beam arranged for pumping wells having inclined wellheads in which torque factors associated with the pumping unit's four-bar linkage are not affected by the modified walking beam.
Another object of the invention is to provide a method and beam pump apparatus for pumping wells having inclined wellheads in which well load is converted to crankshaft torque throughout the pumping cycle at the same rate as with a standard pumping unit design.
Another object of the invention is to provide a method and beam pump apparatus for pumping wells having inclined wellheads in which the polished rod location, speed and acceleration profiles are essentially the same as with the standard vertically aligned pumping unit design.
Another object of the invention is to provide a method and beam pump apparatus having a modified forward walking beam arranged for pumping wells having inclined wellheads in which counterbalance is not affected by the modification and no phase angle mismatch is introduced between the counterbalance torque and well torque curves.